Advances in silicon process technology have led to the development of increasingly smaller sized transistors in integrated circuits. In turn, the decreasing size of transistors has made the circuits increasingly susceptible to damage from ESD events. In order to protect these circuits from damage certain ESD protection schemes have been developed.
Previous approaches to ESD protection include thick-field oxide devices or silicon-controlled rectifier devices coupled to input pads. These devices are effective for protecting signal pads but ineffective for protecting power supply pads because of triggering difficulties and shunting of ESD current through power buses on the integrated circuit (IC) chip that is protected.
Previous approaches also include use of a cantilever diode string connected such that when the voltage on an input pad reaches a certain level, the diodes turn on and create a short circuit between the pad and ground. The cantilever diode string steers the ESD current away from the integrated circuit that could be destroyed by large amounts of current.
Cantilever diode strings alone are not effective when the voltage on the pad being protected is to be switched between multiple voltage levels. On pads where voltages are to be switched, cantilever diode strings alone may respond to each high voltage pulse as if the pulse were an ESD event and couple the pad to ground potential. This would make the high-voltage pad ineffective.
High voltage programming pads such as those found on programmable memory devices are difficult to protect with traditional ESD protection circuits because a high voltage, high current pulse is required to program the memory circuit, and the high current requires low impedance to the circuit receiving the programming pulse. Cantilever diode strings could connect the high voltage programming pulse to potential and prevent the programming pulse from programming the memory circuit.
Cantilever diode strings serve as effective power supply ESD protection without the triggering difficulties of thick-field oxide and silicon-controlled rectifier devices. Because they provide a defined path for ESD current between the pad and ground, cantilever diode strings remove the uncertainty from ESD protection schemes which pass current to a power supply bus. Also, performance of cantilever diode strings does not decrease over time due to use, which allows circuits to survive hundreds or thousands of ESD events.
The standard cantilever diode string as used on low voltage power supply pads cannot be used on pads where fast powering up will occur because each programming pulse is treated as an ESD event and the input voltage is shorted to ground. However, using the novel circuit disclosed herein, appropriate protection can be obtained.